Method and apparatus for inter-technology handoff of a user equipment

ABSTRACT

In order to facilitate a handoff of a communication session between a legacy circuit switched network and a packet data network such as a 3GPP LTE, 3GPP2 UMB, or WiMAX network, which communication session is anchored in an IMS Voice Call Continuity Application Server (VCC AS), a Circuit Switched Proxy (CS Proxy) is provided in the packet data network that communicates with a Mobile Switching Center (MSC) of the circuit switched network. The CS Proxy allows the packet data network to communicate with the VCC AS, the MSC to communicate with the packet data network, and the MSC to communicate with the VCC AS via the packet data network, thereby allowing an exchange of handoff-related information between the packet data network and the VCC AS, the packet data network and the circuit switched network, and the circuit switched network and the VCC AS via the packet data network.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Patent ApplicationNo. 60/944,674, attorney docket no. CE17430T, entitled “Method andApparatus for Inter-Technology Handoff of a User Equipment,” and filedJun. 18, 2007, and further claims priority from U.S. Patent ApplicationNo. 60/972,306, attorney docket no. CE17719T, filed Sep. 14, 2007, andentitled “Method and Apparatus for Inter-Technology Handoff of a UserEquipment,” which applications hereby are incorporated herein in theirentirety.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsystems, and more specifically to handover of a user equipment betweenwireless communication networks implementing different air interfacetechnologies.

BACKGROUND OF THE INVENTION

The evolution of wireless communications has resulted in a proliferationof networks of different technologies and corresponding different airinterfaces. As a result, during the course of a single call, a wirelessuser equipment (UE) may roam among multiple radio access networks(RANs), wherein each such RAN implements a different technology than theother RANs of the multiple RANs, for example, a second generation (2G)circuit switched RAN, such as a GSM (Global System for Mobilecommunications) network providing primarily circuit voice service, and apacket data RAN, such as a later generation 3GPP LTE (Third GenerationPartnership Project Long Term Evolution) network, a 3GPP2 UMB (ThirdGeneration Partnership Project 2 Ultra Mobile Broadband) network, aWiMAX network, or a Wireless Fidelity network based on IEEE 802standards, providing packet switched Voice over Internet Protocol (VoIP)and other data related services.

As the UE roams among the circuit switched RAN and the packet data RANproviding packet data services, it may be beneficial to systemperformance to handoff the UE between the circuit switched RAN and thepacket data RAN. For example, the channel conditions associated with thelatter RAN may be more favorable than the channel conditions associatedwith the former RAN due to such factors as fading, adjacent andco-channel interference, and available power at a serving base station(BS) or radio access network (RAN). By way of another example, anoperator of both a legacy circuit network and a packet data network maydesire to move the UE from one such network to the other network forpurposes of system load balancing. A handoff of a UE from a 2G networkto a 3GPP LTE, 3GPP2 UMB, or WiMAX network is made additionally complexwhen the call involving the UE is currently anchored, and is desired toremain anchored, in an Internet Protocol Multimedia Subsystem (IMS). Asis known in the art, IMS is supposed to be agnostic with respect to theunderlying transport layer over which IMS data is conveyed.

The 3GPP LTE, 3GPP2 UMB, and WiMAX Inter RAN technology handoverstandards are in their infancy. As a result, no method has yet to bedefined for handing off a voice session of a UE between a legacy circuitswitched network, such as a GSM network, and a 3GPP LTE, 3GPP UMB, orWiMAX network. Therefore, a need exists for a method and apparatus forimplementing such a handoff.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless communication system inaccordance with various embodiments of the present invention.

FIG. 2 is a block diagram of a user equipment of FIG. 1 in accordancewith an embodiment of the present invention.

FIG. 3 is a block diagram of a Mobility Management Entity/CircuitSwitched Proxy of FIG. 1 in accordance with an embodiment of the presentinvention.

FIG. 4 is a block diagram of a Mobile Switching Center of FIG. 1 inaccordance with an embodiment of the present invention.

FIG. 5A is a signal flow diagram of a method executed by thecommunication system of FIG. 1 in handing off a voice call from a packetdata network of FIG. 1 to a circuit switched network of FIG. 1 inaccordance with various embodiments of the present invention.

FIG. 5B is a continuation of the signal flow diagram of FIG. 5Adepicting a method executed by the communication system of FIG. 1 inhanding off a voice call from a packet data network of FIG. 1 to acircuit switched network of FIG. 1 in accordance with variousembodiments of the present invention.

FIG. 6 is a block diagram of a Voice Call Continuity Application Serverof FIG. 1 in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

To address the need that exists for a method and apparatus thatimplements a handoff of voice session between a legacy circuit switchednetwork and a packet data network such as a 3GPP LTE, 3GPP2 UMB, orWiMAX network, which voice session is anchored in an IMS Voice CallContinuity Application Server (VCC AS), a Circuit Switched Proxy (CSProxy) is provided in the packet data network that communicates with aMobile Switching Center (MSC) of the circuit switched network. The CSProxy allows the packet data network to communicate with the VCC AS, theMSC to communicate with the packet data network, and the MSC tocommunicate with the VCC AS via the packet data network, therebyallowing an exchange of handoff-related information between the packetdata network and the VCC AS, the packet data network and the circuitswitched network, and the circuit switched network and the VCC AS viathe packet data network.

Generally, an embodiment of the present invention encompasses a methodfor an inter-technology handoff of a communication session from a packetdata network to a circuit switched network. The method includesconveying forward link session traffic to, and receiving reverse linksession traffic from, a user equipment via the packet data network,determining to hand off the communication session to the circuitswitched network, and in response to determining to hand off thecommunication session, conveying, by a CS Proxy of the packet datanetwork to a VCC AS via a CS Proxy/VCC AS interface, informationconcerning a target MSC of the circuit switched network, the target cellidentity, and other information, wherein the CS Proxy conveys suchinformation to the VCC AS without first conveying such information to auser equipment (UE). The method further includes instructing, by the VCCAS via the CS Proxy of the packet data network, the target MSC toprepare for a handover, continuing to anchor the session in the VCC AS,acquiring, by a target Base Station Subsystem (BSS) associated with thetarget MSC, the UE, and providing, by the target BSS, the UE withcommunication session services.

Another embodiment of the present invention encompasses an apparatusthat provides for inter-technology handoff of services between a circuitswitched network and a packet data network. The apparatus comprising aCS Proxy capable of operating in the packet data network and thatcomprises a processor that is adapted to exchange inter-mobile switchingcenter signaling with an MSC of the circuit switched network and toconvey a handover request associated with a UE to an Internet ProtocolMultimedia Subsystem VCC AS via a Circuit Switched Proxy/Voice CallContinuity Application Server interface.

Yet another embodiment of the present invention encompasses a VCC ASthat is adapted to exchange Mobile Application Part (MAP) messaging witha CS Proxy of a packet data network and to receive a handover requestfrom the CS Proxy via a CS Proxy/VCC AS interface.

Turning now to the drawings, the present invention may be more fullydescribed with reference to FIGS. 1-6. FIG. 1 is a block diagram of awireless communication system 100 in accordance with various embodimentsof the present invention. Communication system 100 includes a wirelessuser equipment (UE) 102, for example but not limited to a cellulartelephone, a radiotelephone, or a Personal Digital Assistant (PDA),personal computer (PC), or laptop computer equipped for wireless voicecommunications. UE 102 is capable of engaging in a packet data voicecall with a packet data network 120 and is further capable of engagingin a circuit voice call with a circuit switched network 110, and moreparticularly is capable of communicating with a Radio Access Network(RAN) 122 of the packet data network, preferably via the 3GPP LTE (ThirdGeneration Partnership Project Long Term Evolution) protocol, and with aBSS 112 of the circuit switched network, preferably via a legacyprotocol such as the GSM (Global System for Mobile communications)protocol.

Circuit switched network 110 includes Base Station Subsystem (BSS) 112,which BSS is coupled to a Mobile Switching Center (MSC) 118. BSS 112comprises a transceiver 114, for example, a Base Transceiver Station(BTS), operably coupled to a controller 116, for example, a Base StationController (BSC). MSC 118 includes a call control and mobilitymanagement functionality (not shown) and a switching functionality (notshown).

Packet data network 120 includes a Radio Access Network (RAN) 122, suchas an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) thatcomprises a transceiver 124, such as an evolved Node B, coupled to acontroller 126, such as a Radio Network Controller (RNC). However, inother embodiments of the present invention, the functionality performedby the transceiver and controller may be performed by a single entity.RAN 122 is coupled to a Gateway 138 and a Mobility Management Entity(MME) 134. Preferably, Gateway 138 comprises one or more of a ServingGateway (Serving GWG) (not shown) and a Public Data Network (PDN)Gateway (not shown). Circuit Switched Proxy (CS Proxy) 132 functionalitycan be included in a MME 134 or in other embodiments of the presentinvention CS Proxy 132 may be separate from the MME. CS Proxy 132further is coupled to MSC 118 via an ‘E’ interface, to a Home SubscriberServer (HSS) 152 via an ‘S6x’ interface for retrieving, among otherinformation, Voice Call Continuity Application Server information fromthe HSS, and to a Voice Call Continuity Application Server (VCC AS) 158via a CS Proxy-VCC AS interface (referred to herein as an ‘SGx’interface). CS Proxy 132 emulates an MSC, that is, appears to MSC 118 tobe another MSC and provides circuit switched handover functionalitybetween circuit switched network 110 and packet data network 120. Fromanother perspective, CS Proxy 132 provides circuit switched networkfunctionality in packet data network 120, allowing circuit switchednetwork 110 and packet data network 120 to communicate with each othervia the ‘E’ interface and by use of an inter-system protocol such asMobile Application Part (MAP). CS Proxy 132 further facilitatescommunications between MSC 118 and VCC AS 158 via packet data network120. MME 134 is further coupled to Gateway 138 via an ‘S11’ interfaceand to a Serving GPRS Support Node (SGSN) 136 via an ‘S3’ interface.Gateway 138 is further coupled to SGSN 136 via an ‘S4’ interface and toa Policy Control and Charging Rules Function (PCRF) 140 via an ‘S7’interface. Together, CS Proxy 132, MME 134, SGSN 136, and Gateway 138are collectively referred to herein as an Evolved Packet Core (EPC) 130of packet data network 120.

Each of circuit switched network 110 and packet data network 120 iscoupled to an IP Multimedia Subsystem (IMS) 150. MSC 118 of circuitswitched network 110 is coupled to IMS 150, more particularly to a MediaGateway Control Function (MGCF)/Media Gateway (MGW) 154 of IMS 150 via abearer interface, for example, a Pulse Code Modulation over TimeDivision Multiplexing (PCM over TDM) interface, and a signalinginterface, for example, an ISDN User Part (ISUP) interface. In variousembodiments of the present invention, MGCF/MGW 154 may be implemented ina same network entity or in separate network entities. MME 134 iscoupled to IMS 150, and preferably to a HSS 152 of IMS 150 via an ‘S6a’interface. Gateway 138 has a signaling and bearer connection with IMS150 via an ‘SGi’ interface, and PCRF 140 is coupled to IMS 150 via an‘Rx+’ interface.

IMS 150 includes an IMS core network 156, comprising HSS 152, MGCF/MGW154, and other network entities such as a Call Session Control Function(CSCF) (not shown), and multiple application servers, including a VCC AS158 and a Telephony Application Server (TAS) 160 that are accessible viathe IMS core network. In addition to being coupled to MSC 118, MGCF/MGW154 is further coupled to EPC 130, and in particular to Gateway 138, andprovides a gateway for each of circuit switched network 110 and packetdata network 120 to IMS 150. VCC AS 138 may be coupled to HSS 152 via an‘Sh’ interface. When UE 102 is engaged in a voice call with circuitswitched network 110 that is anchored in IMS 150, MGCF/MGW 154 mayconvert Pulse Code Modulation (PCM) signals received from MSC 118 todata packets, for example, based on a Real Time Protocol/User DatagramProtocol/Internet Protocol (RTP/UDP/IP) protocol suite, for routing toEPC 130 and may convert voice data received from the IMS network to aPCM over TDM (Time Division Multiplex) format for routing to MSC 118.

The above-listed interfaces, with the exception of the ‘E,’ ‘SGx,’ and‘S6x’ interfaces, are all known in the art and will not be described ingreater detail herein. Furthermore, although single interfaces have beendescribed herein between many of the network elements of communicationsystem 100, each interconnection among elements may comprise multipleinterconnections and/or interfaces, such as one or more of a signalinginterface, for example, an interface for an exchange of SIP, ISUP, MAP,or Megaco messages, and a bearer interface or path, such a path for anexchange of voice information.

Each of BSS 112 and RAN 122 provides wireless communication services tousers equipment (UEs) located in a coverage area of the BSS or RAN via arespective air interface 104, 106. Each air interface 104, 106 includesa forward link that includes a pilot channel, at least one forward linktraffic channel, and forward link common and dedicated signalingchannels. Each air interface 104, 106 further includes a reverse linkthat includes at least one reverse link traffic channel, reverse linkcommon and dedicated signaling channels, and an access channel.

Referring now to FIG. 2, an architecture of UE 102 is provided inaccordance with an embodiment of the present invention. UE 102 includesa processor 202, such as one or more microprocessors, microcontrollers,digital signal processors (DSPs), combinations thereof or such otherdevices known to those having ordinary skill in the art, which processoris configured to execute the functions described herein as beingexecuted by UE 102. UE 102 further includes an at least one memorydevice 204, such as random access memory (RAM), dynamic random accessmemory (DRAM), and/or read only memory (ROM) or equivalents thereof,that is coupled to the processor and that maintains data and programsthat may be executed by the associated processor and that allows the UEto perform all functions necessary to operate in communication system100. UE 102 may maintain preprogrammed information in the at least onememory device 204 that facilitates a switching between networks 110 and120.

Referring now to FIG. 3, an architecture of MME 134/CS Proxy 132 isprovided in accordance with an embodiment of the present invention. TheMME/CS Proxy include a processor 302, such as one or moremicroprocessors, microcontrollers, digital signal processors (DSPs),combinations thereof or such other devices known to those havingordinary skill in the art, which processor is configured to execute thefunctions described herein as being executed by the MME/CS Proxy. TheMME/CS Proxy further include an at least one memory device 304 that maycomprise random access memory (RAM), dynamic random access memory(DRAM), and/or read only memory (ROM) or equivalents thereof, thatmaintain data and programs that may be executed by the associatedprocessor and that allow the MME/CS Proxy to perform all functionsnecessary to operate in communication system 100. At least one memorydevice 304 further maintains configuration information of neighboringMSCs, such as MSC 118, for facilitating handoffs to or from such MSCs.For example, such configuration information may include routinginformation for routing information to such MSCs, protocols supported bysuch MSCs, and identifiers of the BSSs supported by such MSCs, such asidentifiers of the transceivers and/or controllers included in the BSSs.Preferably, CS Proxy 132 is implemented by processor 302 based onprograms and data maintained by at least one memory device 304. However,in another embodiment of the present invention, CS Proxy 132 may beincluded in a network entity separate from MME 134. In the latterinstance, CS Proxy 132 includes its own processor and associated atleast one memory device that maintains data and programs that may beexecuted by the processor and that allow the CS Proxy to perform allfunctions necessary to operate in communication system 100, and theconfiguration information described above as being maintained by MME 132may, instead, be maintained by the at least one memory device of the CSProxy or may be distributed between the MME and the CS Proxy.

Referring now to FIGS. 4 and 6, architectures of MSC 118 and VCC AS 158are provided in accordance with an embodiment of the present invention.Each of MSC 118 and VCC AS 158 includes a respective processor 402, 602,such as one or more microprocessors, microcontrollers, digital signalprocessors (DSPs), combinations thereof or such other devices known tothose having ordinary skill in the art, which processor is configured toexecute the functions described herein as being executed by MSC 118 andVCC AS 158. Each of MSC 118 and VCC AS 158 further includes a respectiveat least one memory device 404, 604 such as random access memory (RAM),dynamic random access memory (DRAM), and/or read only memory (ROM) orequivalents thereof, that is coupled to the processor and that maintainsdata and programs that may be executed by the associated processor andthat allows the MSC and VCC AS to perform all functions necessary tooperate in communication system 100.

The functionality described herein as being performed by UE 102, MME134, CS Proxy 132, MSC 118, and VCC AS 158 is implemented with or insoftware programs and instructions stored in the respective at least onememory device 204, 304, 404, 604 associated with the UE, MME, CS Proxy,MSC, and VCC AS and executed by a processor 202, 302, 402, 602associated with the UE, MME, CS Proxy, MSC, and VCC AS. However, one ofordinary skill in the art realizes that the embodiments of the presentinvention alternatively may be implemented in hardware, for example,integrated circuits (ICs), application specific integrated circuits(ASICs), and the like, such as ASICs implemented in one or more of theUE, MME, CS Proxy, MSC, and VCC AS. Based on the present disclosure, oneskilled in the art will be readily capable of producing and implementingsuch software and/or hardware without undo experimentation.

In order for UE 102 to engage in a circuit services voice call or apacket data voice call via circuit switched network 110 or packet datanetwork 120, respectively, each of UE 102, circuit switched network 110,and packet data network 120 operates in accordance with known wirelesstelecommunications protocols. Circuit switched network 110 preferably isa legacy communication system that provides circuit switchedcommunication services to subscribers serviced by the network (it mayalso provide packet data services) and that operates in accordance withthe GSM standards or later generations of the GSM standards. Packet datanetwork 120, as illustrated herein, is a 3GPP LTE communication systemthat provides packet data communication services to subscribers servicedby the network; however, other applicable packet data networks include3GPP2 UMB and WiMAX packet data networks. To ensure compatibility, radiosystem parameters and call processing procedures are specified by thestandards, including call processing steps that are executed by an UEand a base station subsystem or other access network serving the UE andbetween the base station subsystem or other access network andassociated infrastructure. However, those of ordinary skill in the artrealize that packet data network 120 may operate in accordance with anyone of a variety of wireless packet data communication systems thatsupports multimedia packet data-based communication sessions, such asthe IEEE (Institute of Electrical and Electronics Engineers) 802.xxstandards, for example, the 802.11, 802.15, or 802.16 or 802.20standards, and that circuit switched network 110 may operate inaccordance with any one of a variety of well-known legacy wirelesstelecommunication systems that provide circuit switched communicationservices.

In communication system 100, when UE 102 is engaged in a voice call withone of circuit switched network 110 and packet data network 120, the UEmay roam through the system. As a result of the roaming, situations mayarise where it is desirable to hand off UE 102 from one of circuitswitched network 110 and packet data network 120 to the other network.For example and as is known in the art, while roaming in communicationsystem 100 and being serviced by BSS 112, UE 102 may receive a strongersignal from RAN 122. And similarly, while roaming in communicationsystem 100 and being serviced by RAN 122, UE 102 may receive a strongersignal from BSS 112. Typically signal strengths are determined by a UE,such as UE 102, measuring a pilot channel associated with the RAN orBSS. When a pilot channel of a serving RAN or BSS is weaker than athreshold value and a pilot channel of another RAN or BSS, thattypically indicates a desirability of a handoff.

By way of another example, the costs associated with operating UE 102 oncircuit switched network 110 may be different from the costs associatedwith operating UE 102 on packet data network 120. In turn, an operator(or operators) of networks 110 and 120 may charge a different fee foruse of each network. As a result, a user of UE 102 may set a userpreference to operate on the lower cost network whenever the UE is ableto obtain a traffic channel in the lower cost network. When UE 102 isengaged in a voice call in a higher cost network and is able to obtain atraffic channel in the lower cost network, the UE, or the user of the UEif the user is informed of the availability of a traffic channel in thelower cost network, may initiate a handoff to the lower cost network. Byway of yet another example, it may be desirable to move a UE, such as UE102, that is actively engaged in a voice call in circuit switchednetwork 110 to packet data network 120 when the user of UE 102 prefersto use video telephony service rather than a voice call, and packet datanetwork 120 supports video telephony but circuit switched network 110does not. By way of still another example, for load balancing purposes,for network cost consideration purposes, or due to a need to cleartraffic channels in a coverage area in order to facilitate emergencycommunications, an operator of a communication system such ascommunication system 100 may find it desirable to move an UE, such as UE102, that is actively engaged in a voice call in one of circuit switchednetwork 110 to the packet data network 120 or vice versa.

In order to facilitate a handoff of a UE, such as UE 102, communicationsystem 100 provides a method and apparatus for a handoff of the UE frompacket data network 120 to circuit switched network 110 when the UE isactively engaged in a communication session, which may comprise a voicecall, and/or a non-voice call, such as video, in either network.

Referring now to FIGS. 5A and 5B, a signal flow diagram 500 is providedthat illustrates a method executed by communication system 100 inhanding off UE 102 from packet data network 120 to circuit switchednetwork 110 in accordance with various embodiments of the presentinvention. Signal flow diagram 500 begins when UE 102 is engaged in anIMS telephony session (501-504) via packet data network 120 that iscontrolled by IMS 150, and in particular is anchored at VCC AS 158. Atthis point a packet switched bearer path has been established between UE102 and the MGCF/MGW 154 via RAN 122 and EPC 130, along with a circuitswitched bearer path between the MGCF/MGW 154 and the remote party (notshown).

Sometime prior to initiating the IMS telephony session (501-504), UE 102initiates an Attach procedure (not shown) with the packet data network120 followed by an IMS Registration procedure (not shown) with the IMSnetwork 150. As a result of performing the Attach procedure, the UE 102is authenticated and packet data network encryption keys are generatedand stored in both the MME 134 and UE 102. The preferred method foracquiring the CS encryption key (Kc) needed to perform a handover to thecircuit switched network 110 is to use a hash function on the packetdata encryption keys stored in the MME 134 and UE 102 to generate aVCC-only CS encryption key (Kc). Following authentication of the UE 102,the MME 134 performs an Update Location procedure during whichsubscription data is provided by the HSS 152 to the MME 134, includingthe UE's Mobile Subscriber ISDN Number (MSISDN) and the identity of theVCC AS 158 serving the UE 102. In another embodiment of the presentinvention, the CS Proxy 132 may support an S6x interface for retrievingthe UE's MSISDN and the VCC AS identity information from the HSS 152. Asa result of performing the IMS Registration procedure, the VCC AS 158obtains via 3^(rd) party registration the MSISDN, ‘tel’ URI and otherPublic User Identities (PUIs) in the UE's implicit registration set.

While UE 102 is in an active communication session, such as a voicecall, with a remote party (not shown) via packet data network 120, theUE monitors (506) qualities, in particular a signal strength oralternatively any of a variety of other signal qualities such as asignal-to-noise ratio (SNR), a carrier-to-interference ratio (C/I),pilot power-to-total power (Ec/Io) ratio, a bit error rate (BER), or aframe error rate (FER), of pilots associated with each of RAN 122 ofpacket data network 120 and BSS 112 of circuit switched network 110. UE102 may monitor the pilots of each network 110, 120 concurrently or mayswitch between networks in monitoring the pilots. UE 102 mayself-determine when or whether to monitor the pilots associated with BSS112 of circuit switched network 110 or may monitor the pilots inresponse to receiving an instruction to do so from packet network 120.

UE 102 reports (506) the monitored pilot(s) in accordance with wellknown reporting procedures. For example, when a quality of a pilot ofcircuit switched network 110, and more particularly of air interface104, is measured by UE 102 when operating in packet data network 120 andexceeds an inter-system handoff threshold, or a quality of a previouslyreported pilot of packet data network 120, and more particularly of airinterface 106, is measured by UE 102 when operating in packet datanetwork 110 and falls below an intra-system handoff threshold, the UEmay report the monitored pilot(s) back to RAN 122, and in particular tocontroller 126. Controller 126 then stores the reported pilotmeasurements.

Based on the pilot measurements associated with RAN 122 and BSS 112 andreported by UE 102, packet data network 120, and in particular RAN 122,may determine (508) to handoff UE 102 to circuit switched network 110and BSS 112. For example, when a pilot of RAN 122 compares unfavorablyto (is below, in the case of a signal strength threshold) theintra-system handoff threshold and/or a pilot of BSS 112 comparesfavorably to (exceeds, in the case of a signal strength threshold) theinter-system handoff threshold, this may indicate a desirability of ahandoff. By way of another example, costs associated with operating UE102 on network 110 may be different from the costs associated withoperating UE 102 on network 120. In turn, an operator (or operators) ofnetworks 110 and 120 may charge a different fee for use of each network.If circuit switched network 110 is the lower cost network, a user of UE102 may program into the UE a directive to operate on the circuitswitched network 110 whenever a measurement of a pilot associated withthe circuit switched network compares favorably to the inter-systemhandoff threshold. By way of still another example, for load balancingpurposes, for network cost consideration purposes, or due to a need toclear traffic channels in a coverage area in order to facilitateemergency communications, an operator of communication system 100 mayfind it desirable to move a UE, such as UE 102, that is actively engagedin a communication session in packet data 120 to circuit switchednetwork 110 whenever a measurement of a pilot associated with thecircuit switched network compares favorably to the inter-system handoffthreshold.

In response to determining to handoff UE 102 from packet data network120 and RAN 122 to circuit switched network 110 and BSS 112, RAN 122assembles a request to handoff the communication session that includesthe measurements, that identifies a target BSS, that is, BSS 112, andthat further identifies the UE to be handed off, that is, UE 102. RAN122 then conveys (510) this request to EPC 130, and in particular to MME134. Based on the received request and with reference to theconfiguration information maintained by the MME 134, the MME, and moreparticularly CS Proxy 132, determines (512) the target MSC, that is MSC118, and assembles, and conveys (514) a handover request to VCC AS 158via the SGx interface that includes the target cell or sectoridentifier, the identifier of target MSC 118, the VCC AS PSI (PublicService Identity), the CS Proxy identifier, the MSISDN and theencryption information which is required to handover to the circuitswitched network, or domain, including a CS encryption key (Kc) and thesupported/allowed A5 encryption algorithms. The CS Proxy identifieridentifies the CS Proxy that the VCC AS is to talk to, that is, CS Proxy132, since, in the prior art, a VCC AS is unable to talk directly to anMME due to protocol incompatibilities. Preferably, the handover requestis a 2G (Second Generation) circuit switched Handover Required message,that further may be encapsulated in an IP data packet.

The VCC AS 158 uses the MSISDN included in the handover request from CSProxy 132 to associate the handover request with the IMS sessiontelephony it is anchoring for the UE 102 (516). Further, in response toreceiving the handover request, VCC AS 158 instructs MSC 118, via CSProxy 132, to prepare for a handover. That is, in response to receivingthe handover request, VCC AS 158 assembles, and conveys (518) to CSProxy 132, a MAP PREPARE-HANDOVER message. MAP is a circuit switchnetwork protocol that may be used for inter-MSC communications. Byincluding CS Proxy 132 in packet data network 120, each of VCC AS 158and MSC 118 is able to talk to the packet data network, and to CS Proxy132 in particular, and further are able to talk to each other via thepacket data network.

The MAP PREPARE-HANDOVER message assembled by VCC AS 158 includes the CSProxy identifier, the MSC identifier, the target cell or sectoridentifier and the encryption information. In response to receiving theMAP PREPARE-HANDOVER message, CS Proxy 132 conveys (520) a MAPPREPARE-HANDOVER message to target MSC 118 that includes the CS Proxyidentifier, target cell or sector identifier and the encryptioninformation. In one embodiment of the present invention, CS Proxy 132may forward the MAP PREPARE-HANDOVER message received from VCC AS 158,with or without the CS Proxy identifier; however, in another embodimentof the present invention, CS Proxy 132 may assemble and convey anotherMAP PREPARE-HANDOVER message that includes some or all of theinformation received from VCC AS 158.

In response to receiving the MAP PREPARE HANDOVER message, MSC 118assembles and conveys (522) a handover request, preferably a HO-REQUESTmessage, to target BSS 112 and, in response, receives (524) anacknowledgement of the request from the BSS. MSC 118 further conveys(526) a MAP PREPARE HANDOVER RESPONSE message to CS Proxy 132 thatincludes a handover number (HO#) for establishment of a circuitconnection between the MSC and MGCF/MGW 154, in particular the MGW, anda handover command (HO_CMD). In response to receiving the MAPPREPARE-HANDOVER-RESPONSE message, CS Proxy 132 conveys (528) a MAPPREPARE-HANDOVER-RESPONSE message to VCC AS 158 that includes the VCC ASidentifier, the handover number, and the handover command. In oneembodiment of the present invention, CS Proxy 132 may forward the MAPPREPARE-HANDOVER-RESPONSE message received from target MSC 118; however,in another embodiment of the present invention, CS Proxy 132 mayassemble, and convey to the VCC AS, another MAPPREPARE-HANDOVER-RESPONSE message that includes some or all of theinformation received from the MSC.

In response to receiving the MAP PREPARE-HANDOVER-RESPONSE message fromCS Proxy 132, VCC AS 158 establishes (530-560) a circuit connection withtarget MSC 118 and, via the MSC, a connection to UE 102 via circuitswitched network 110. More particularly, in response to receiving theMAP PREPARE-HANDOVER-RESPONSE message from CS Proxy 132, VCC AS 158assembles, and conveys (530) to MGCF/MGW 154, a session invitation,preferably a SIP INVITE message, that includes the handover number androuting identifiers associated with UE 102 and the remote party to thecommunication session. In response to receiving the session invitation,MGCF/MGW 154 provides (532) target MSC 118 with the handover number andreserves trunk circuits for the handoff of the communication session viaan exchange of ISUP IAM (Initial Address Message) and ISUP ACM (AddressComplete Message) messages. In response to receiving the handovernumber, target MSC 118 informs (534) VCC AS 158 that the handover isunderway, preferably by conveying a SIP PROGRESS message to the VCC AS.In response to receiving the SIP PROGRESS message, VCC AS 158 notifies(536-538) UE 102 that resources are being assigned to the UE in circuitswitched network 110, and more particularly at target BSS 112, byconveying a Handover Command (HO_CMD) message, that includes a commandto handover to target BSS 112, to the UE via packet data network 120,that is, via EPC 130, in particular via CS Proxy 132 and MME 134, andRAN 122.

In response to receiving the handover command, UE 102 attempts to access(540) target BSS 112. In response to receiving the access attempt,target BSS 112 acquires UE 102 and allocates resources at the BSS to theUE 102. Further, target BSS 112 notifies (542) target MSC 118 that theBSS has detected UE 102 and that acquisition of the UE is underway,preferably by conveying a HO-DETECTED message to the MSC. In response,target MSC 118 informs (544, 546) VCC AS 158, via packet data network120, of the detection of UE 102 by circuit switched network 110. Thatis, target MSC 118 informs (544) CS Proxy 132 of the detection of UE 102by target BSS 112, that is, of the access attempt by the UE, and thatacquisition of the UE is underway, preferably by conveying a MAPPROGRESS-ACCESS-SIGNAL REQUEST message to the CS Proxy. CS Proxy 132then informs (546) VCC AS 158 of the access attempt and acquisition ofthe UE, preferably by forwarding the received MAP PROGRESS-ACCESS-SIGNALREQUEST message to the VCC AS.

When target BSS 112 has completed acquisition of UE 102 and providescommunication session-related services to the UE, UE 102 informs (548)BSS 112 that the UE has been successfully acquired by the BSS,preferably by conveying a handover complete (HO-COMPLETE) message to theBSS. Alternatively, target BSS 112 may self-determine that the UE hasbeen successfully acquired. BSS 112 then informs (550) target MSC 118 ofthe successful acquisition of the UE, preferably by conveying aHO-COMPLETE message to the MSC. In response to receiving the HO-COMPLETEmessage, target MSC 118 connects the circuit with UE 102 through theMSC. Further, in response to acquiring UE 102, target MSC 118 informs(552, 554) VCC AS 158, via packet data network 120, of the successfulacquisition of UE 102 by circuit switched network 110 and triggers theclearing of resources in packet data network 120. That is, target MSC118 informs (552) CS proxy 132 of the successful acquisition of UE 102by the BSS and triggers a clearing of resources in packet data network120 by conveying a MAP SEND-END-SIGNAL REQUEST message to the CS Proxy.In turn, CS Proxy 132 informs (554) VCC AS 158 of the successfulacquisition of UE 102 by circuit switched network 110 and triggers theclearing of resources in packet data network 120 by forwarding the MAPSEND-END-SIGNAL REQUEST message to the VCC AS.

In addition, in response to being informed of the successful acquisitionof UE 102 by target BSS 112, target MSC 118 informs (560) MGCF/MGW 154of the successful acquisition of the UE, preferably by conveying an ISUPANSWER message to the MGCF/MGW. MGCF/MGW 154 then informs (562) VCC AS158 of the acceptance of the handover by circuit switched network 110,preferably by conveying a SIP 200 OK message in response to the sessioninvitation received by the MGCF/MGW from the VCC AS.

At this point, UE 102 has switched over to circuit switched network 110.As a result, it is desirable to tear down resources reserved to the UE102 in packet data network 120. Therefore, in response to being informedof the acceptance of the handover by circuit switched network 110, VCCAS 158 arranges, via the Circuit Switched Proxy, a release of resourcesin packet data network 120. That is, VCC AS 158 negotiates (564), via CSProxy 132, a release of resources allocated to UE 102 in packet datanetwork 120. CS Proxy 132, via MME 134, then negotiates (566) a releaseof packet data network resources with RAN 122.

In response to receiving the SIP 200 OK message (562), VCC AS 158instructs (572) MGCF/MGW 154 to connect the bearer path of thecommunication session with circuit switched network 110 and to switch toa circuit switched communication session conversion scheme by conveyinga SIP RE-INVITE message to the MGCF/MGW. In response to receiving theSIP RE-INVITE message, MGCF/MGW 154 connects the bearer path of thecommunication session with circuit switched network 110, switches to acircuit switched communication session conversion scheme, and informs(574) VCC AS 158 of the switch and conversion by conveying a SIP 200 OKmessage to the VCC AS. At this point, a circuit switched bearer path hasbeen established (576-579) between UE 102 and the remote party (notshown) via target BSS 112, target MSC 118, and MGCF/MGW 154 and callflow diagram 500 then ends.

By providing a CS Proxy 132 in packet data network 120 that is coupledto MSC 118 of circuit switched network 110, communication system 100permits the MSC to communicate with the packet switched network andfurther with an IMS 150 VCC AS 158 via the packet data network. Thus theMSC and VCC AS may exchange circuit switched network messages, and inparticular circuit switched network handover messages such as MAPmessages, via the packet switched network prior to connections beingestablished in the circuit switched network for the UE being handed off.By permitting MSC 118 and VCC AS 158 to exchange messaging via thepacket switched network, communication system 100 facilitates anexchange of handoff-related information between the VCC AS anchoring thecall and the target circuit switched network 110, thereby allowing theVCC AS to control the set up of the communication session in the targetcircuit switched network in preparation for handover.

By including a CS Proxy 132 in packet data network 120, communicationsystem 100 allows the packet data network to communicate with VCC AS 158of IMS 150, MSC 118 of circuit switched network 110 to communicate withthe packet data network, and the MSC to communicate with the VCC AS viathe packet data network, thereby facilitating an exchange ofhandoff-related information between the packet data network and the VCCAS, the packet data network and the circuit switched network, and thecircuit switched network and the VCC AS via the packet data network.

While the present invention has been particularly shown and describedwith reference to particular embodiments thereof, it will be understoodby those skilled in the art that various changes may be made andequivalents substituted for elements thereof without departing from thescope of the invention as set forth in the claims below. Accordingly,the specification and figures are to be regarded in an illustrativerather then a restrictive sense, and all such changes and substitutionsare intended to be included within the scope of the present invention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any variation thereof, are intended tocover a non-exclusive inclusion, such that a process, method, article,or apparatus that comprises a list of elements does not include onlythose elements but may include other elements not expressly listed orinherent to such process, method, article, or apparatus. The terms‘including’ and/or ‘having’, as used herein, are defined as comprising.Furthermore, unless otherwise indicated herein, the use of relationalterms, if any, such as first and second, top and bottom, and the likeare used solely to distinguish one entity or action from another entityor action without necessarily requiring or implying any actual suchrelationship or order between such entities or actions. An elementpreceded by “ . . . a” does not, without more constraints, preclude theexistence of additional identical elements in the process, method,article, or apparatus.

1. A method for inter-technology handoff of a communication session froma packet data network to a circuit switched network, the methodcomprising: conveying forward link session traffic to, and receivingreverse link session traffic from, a user equipment via the packet datanetwork; determining to hand off the communication session to thecircuit switched network; in response to determining to hand off thecommunication session, conveying, by a Circuit Switched Proxy of thepacket data network to a Voice Call Continuity Application Server via aCircuit Switched Proxy/Voice Call Continuity Application Serverinterface, information concerning a target Mobile Switching Center ofthe circuit switched network, the target cell identity, and otherinformation; instructing, by the Voice Call Continuity ApplicationServer via the Circuit Switched Proxy of the packet data network, thetarget Mobile Switching Center to prepare for a handover; continuing toanchor the session in the Voice Call Continuity Application Server;acquiring, by a target Base Station Subsystem associated with the targetMobile Switching Center, the user equipment; and providing, by thetarget Base Station Subsystem, the user equipment with communicationsession services.
 2. The method of claim 1, further comprisingconveying, by the Mobile Switching Center, handover-related informationto the Voice Call Continuity Application Server via the Circuit SwitchedProxy.
 3. The method of claim 2, wherein conveying, by the MobileSwitching Center, handover-related information to the Voice CallContinuity Application Server via the circuit switched proxy comprisesconveying a Mobile Application Part message by the Mobile SwitchingCenter to the Circuit Switched Proxy and conveying a Mobile ApplicationPart message by the Circuit Switched Proxy to the Voice Call ContinuityApplication Server.
 4. The method of claim 1, wherein instructingcomprises conveying a Mobile Application Part message by the Voice CallContinuity Application Server to the Circuit Switched Proxy andconveying a Mobile Application Part message by the Circuit SwitchedProxy to the target Mobile Switching Center.
 5. The method of claim 1,further comprising notifying the user equipment, by the Voice CallContinuity Application Server via the packet data network of theassignment of resources to the User Equipment in the circuit switchednetwork.
 6. The method of claim 1, further comprising notifying theVoice Call Continuity Application Server, by the target Mobile SwitchingCenter via the Circuit Switched Proxy, of the acquisition of the userequipment by the circuit switched network.
 7. The method of claim 6,further comprising, in response to receiving notification of theacquisition of the user equipment by the circuit switched network,arranging, by the Voice Call Continuity Application Server via theCircuit Switched Proxy, a release of resources in the packet datanetwork.
 8. The method of claim 6, further comprising instructing, bythe Voice Call Continuity Application Server, a gateway to connect abearer path of the communication session with the circuit switchednetwork.
 9. An apparatus that provides for inter-technology handoff ofservices between a circuit switched network and a packet data network,the apparatus comprising a Circuit Switched Proxy capable of operatingin the packet data network and that comprises a processor that isadapted to exchange inter-mobile switching center signaling with aMobile Switching Center of the circuit switched network and to convey ahandover request associated with a user equipment to an InternetProtocol Multimedia Subsystem Voice Call Continuity Application Servervia a Circuit Switched Proxy/Voice Call Continuity Application Serverinterface.
 10. The apparatus of claim 9, wherein the Circuit SwitchedProxy is implemented by a Mobility Management Entity.
 11. The apparatusof claim 9, wherein the Circuit Switched Proxy is implemented as a standalone network entity with interface to Mobility Management Entity,Mobile Switching Center, and Internet Protocol Multimedia Subsystem. 12.The apparatus of claim 9, wherein the Circuit Switched Proxy supports anE interface for the exchange of inter-mobile switching center signalingwith the Mobile Switching Center.
 13. The apparatus of claim 9, whereinthe Circuit Switched Proxy supports an SGx interface for the exchange ofhandover-related signaling with the Internet Protocol MultimediaSubsystem.
 14. The apparatus of claim 9, wherein the Circuit SwitchedProxy supports an S6x interface for retrieving Voice Call ContinuityApplication Server and User Equipment identification information fromthe Home Subscriber Server.
 15. The apparatus of claim 9, wherein theprocessor is adapted to exchange inter-mobile switching center signalingand handover-related signaling via Mobile Application Part messaging.16. A Voice Call Continuity Application Server that is adapted toexchange Mobile Application Part messaging with a Circuit Switched Proxyof a packet data network and to receive a handover request from theCircuit Switched Proxy via a Circuit Switched Proxy/Voice CallContinuity Application Server interface.
 17. The Voice Call ContinuityApplication Server of claim 16, wherein the Voice Call ContinuityApplication Server is further adapted to, as part of a handover of auser equipment from a packet data network to a circuit switched networkand in response to receiving notification of the acquisition of the userequipment by the circuit switched network, arrange, via the CircuitSwitched Proxy, a release of resources in the packet data network. 18.The Voice Call Continuity Application Server of claim 17, wherein theVoice Call Continuity Application Server is further adapted to, as partof a handover of a user equipment from a packet data network to acircuit switched network and in response to receiving notification ofthe acquisition of the user equipment by the circuit switched network,instruct a gateway to connect a bearer path of the communication sessionwith the circuit switched network.